BEIJING - Droughts and floods stoked by global warming threaten to destabilize China's grain production, the nation's top meteorologist has warned, urging bigger grain reserves and strict protection of farmland and water supplies.

Extreme weather damage can now cause annual grain output in China, the world's biggest grain producer, to fluctuate by about 10 to 20 percent from longer-term averages.

But with global warming intensifying droughts, floods and pests, the band of fluctuation in annual production could widen to between 30 and 50 percent, Zheng Guoguang, head of the China Meteorological Administration, wrote in a new essay. He did not say how long it might be before that could happen.

A stretch of especially bad weather for farming conditions could be disastrous for the world's most populous nation, Zheng wrote in the latest issue of Seeking Truth (Qiushi), the ruling Communist Party's main magazine, which was published on Tuesday and reached subscribers on Wednesday.

If extreme climatic disasters occur twice or more within five years -- for example, major drought over two or three years -- then the impact on our country's economic and social development would be incalculable, wrote Zheng, who plays a role in developing China's climate change policies.

Zheng's warning appeared days before governments gather in Copenhagen seeking to forge the framework of a new agreement on fighting global warming.

As the world's biggest greenhouse gas emitter, China will be a crucial player in those talks. Last week the government announced emissions goals for the next decade.

Zheng's blunt words underscored the hard choices facing Beijing, as both a big polluter and a vulnerable victim of global warming. He is a member of a leading small group charged with developing the government's policies on climate change.

FARMING POPULATION

A vast developing country with a farming population of some 750 million, China is also one of the nations most vulnerable to global warming, wrote Zheng. He urged greater attention to adapting to unstoppable shifts in temperatures, rainfall and extreme weather.

China should make a priority of reducing the impact of global warming on the country's food security, and strengthening the capacity of agriculture to withstand climatic risks, wrote Zheng.

China's grain production has recently reached record levels, despite damage from droughts, floods and frost. In 2008, China enjoyed a fifth straight year of bumper harvests, with grain output at a record 525 million tonnes. U.S. output over the 2007-08 growing year was 412 million tonnes.

Citing previously published research, Zheng wrote that by 2030, China's crop productivity could be 5 to 10 percent lower than it would be without global warming.

While rising temperatures may extend potential growing times and areas for some crops, especially in northeast China, the accompanying rise in evaporation rates is likely to reduce water supplies, undercutting any increases in crop yields, wrote Zheng.

Without adequate adaptive measures, in the second half of the century wheat, rice and corn production could fall by as much as 37 percent of recent averages, he wrote, citing earlier research.

But China cannot depend on the international marketplace to make up for these potential shortfalls, because global warming would also erode farming productivity in many parts of the globe, Zheng wrote.

Instead, the government should focus on expanding domestic grain reserves, protecting farmland, developing water-saving technology for farms, and boosting farmers' productivity, he wrote.

(Editing by Ken Wills and Alex Richardson)

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